Perhaps an explanation may lay in something like "less surface area for more material". The threads in the middle of the rope are through friction supported by the threads surrounding them and therefore seem stronger? Could be, dunno. Still, as the tip is the weakest point, the strength of the furled leader isn't that interesting to me. Might have something to do with fishing line being bought here(Netherlands) by diameter and over there(England/US) by lbs breaking strength.

Karel